The present invention relates to a semiconductive silicone rubber roller for office instruments utilizing a photocopying unit used around the photosensitive drums or, in particular, as a developing roller having a surface covering layer exhibiting good balance between anti-wearing resistance and electrostatic chargeability which retains the initial developing performance over a long service life.
In the electrostatic recording instruments such as copying machines, laserbeam printers, facsimile machines and the like among office-automation instruments working as a type of photocopier, the step of image development is conducted in the following manner. A thin layer of a triboelectrically chargeable toner formed on a developing rubber roller is brought into contact with a photosensitive body to effect development of a latent image on the photosensitive body. The toner particles are charged with static electricity of a specified level by means of the triboelectric charging mainly between the toner particles and the developing roller and between the toner particles and a developing blade and, after adjustment of the thickness of the toner layer by the developing blade press-contacted with the developing roller, transferred onto the surface of the photosensitive body and deposited by the electrostatic force on the electrostatic latent image formed on the photosensitive body to exhibit a visible image. The requirements for the properties of the developing roller include excellent durability for long-term service, appropriate surface roughness, uniform thickness of the toner layer formed thereby, stability in electrostatic charging behavior and so on.
One of the criteria for the evaluation of the durability of the developing roller is given by the occurrence of scratches formed on the surface of the roller after service over a long term. In addition to direct rubbing with the toner particles, scratches are also possibly formed by rubbing with the toner particles bonded to the surface of the blade. Since the thickness of the toner layer on the developing roller is about 20 to 30 xcexcm, scratches formed on the surface of the roller, even when the depth thereof is very small, 4 to 5 xcexcm, locally affect the surface roughness and electrostatic charging characteristics. This leads to troubles relative to the quality of the developed images, such as fogging and the appearance of white voids. Accordingly, it is important that the material of the surface-covering layer of the developing roller satisfies the requirements for the stability of the outer diameter of the roller, surface roughness and electrostatic charging characteristics.
Further, the developing roller plays a role for triboelectric charging of the toner particles coming into contact with the roller surface. In the triboelectric charging of two different materials of different natures, the surfaces of the materials after contacting and separating are charged with electrostatic charges of equal quantity but of different signs. The sign of the charges, which may be positive or negative on one and negative or positive on the other, is determined by the relative natures of the two materials or so-called contact-electrification ranks of the materials. The charging capacity of the materials is determined by the electrostatic capacity inherent in the respective materials. Since the condition of electrostatic charging depends on the relative natures of the contacting two materials, the charging characteristics, e.g., sign of charges, quantity of charging, stability of charging, etc., of one of the contacting materials have great influence on the charging characteristics of the other. Namely, the charging characteristics of the developing roller are deeply correlated with the charging quantity of the triboelectrically chargeable toner particles, thickness of the toner layer, toner consumption and toner releasability within the developing unit and with the full-black density, i.e. Macbeth density, occurrence of fogging and voids on the actually printed images. It is therefore important to select a rubbery material for the surface layer of the roller capable of keeping constancy of the quantity of electrostatic charges in relation to the selection of the filler, compounding technology and preparation and coating technology of the coating composition.
It is conventional that the surface layer of developing rollers is formed from a silicone rubber by virtue of the excellent and advantageous properties thereof. For example, silicone rubbers exhibit, as compared with other types of rubbery materials, stable elasticity over a long period with excellent weatherability to withstand adverse environmental conditions. A sharp charge distribution of triboelectrically charged toner particles can be obtained on a roller surface made from a silicone rubber. Further, the roller surface of a silicone rubber has a low susceptibility to the occurrence of filming by the toner particles. Even with these advantageous properties of a silicone rubber-made developing roller, troubles have been encountered in recent years such as the occurrence of fogging and insufficient printing density caused by repeated printing along with the trend toward higher and higher printing velocity necessitating an increase in the load on the developing rollers. These troubles are caused presumably by the streaky scratches running around the circumference of the developing rollers and an eventual change in the surface roughness.
Though advantageous as a material of developing rollers as is described above, silicone rubbers are generally not quite satisfactory in respect of their abrasion resistance. According to the results of a running test by using a machine for actual service, some of the silicone rubber rollers after long-term service or after expiration of the cartridge life, exhibited a decrease in the outer diameter of the roller due to wearing of the surface and a decrease or increase in the surface roughness along with changes in the electrostatic charging characteristics.
In view of the above mentioned defects of silicone rubbers, proposals were made for the use of a urethane rubber having superior mechanical properties as a material of developing rollers as compared with silicone rubbers. Urethane rubber-made developing rollers, however, have problems caused by changes in their elastic modulus and electric properties such as volume resistivity in long-term service because urethane rubbers are subject to a hydrolysis reaction with moisture.
The development work for the developing rollers in recent years has been directed to these problems, in compliance with the requirement for upgrading the image quality which can be accomplished by further and further decreasing the thickness of the toner layer on the surface of the developing roller, to minimize changes in the developing characteristics due to the instability of the physical and morphological conditions of the roller surface.
It is now understood that the above mentioned problems can hardly be solved with an elastic roller formed from a single rubbery material. Accordingly, it is eagerly desired to develop a developing roller of the separate-function type comprising an elastic base layer on and around a shaft made from a rubbery material having high stability over a long time of its elastic modulus and volume resistivity and a cladding layer formed thereon from a material having excellent abrasion resistance and moderate charging characteristics in combination which is imparted with desirable electrostatic charging characteristics by compounding with a dielectric filler.
The present invention accordingly has an object to provide an improved developing roller for a photocopying unit having a composite structure of the rubber portion comprising of a base layer on a metallic shaft made from a rubber having high resistance to withstand adverse environmental conditions and a cladding layer thereon formed from an elastic material having a well balanced combination of abrasion resistance and electrostatic charging characteristics so that the initial high performance for image development can be lastingly retained over a long time.
Thus, the developing roller provided by the present invention is an integral body which comprises:
(a) a shaft made from an electroconductive material such as a metal;
(b) a base layer of a semiconductive silicone rubber formed on and around the shaft; and
(c) a cladding layer of a synthetic resin composition containing dielectric particles dispersed therein formed on and around the base layer, the synthetic resin being selected from urethane resins and acrylic melamine resins.
In particular, the dielectric particles are positively polarizable and the material of the dielectric particles preferably has a dielectric constant in the range from 2.0 to 8.0.